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((Short description|Submerged ice anchored to a river bottom or seafloor))
((refimprove|date=November 2012))
[[Image:Anchor ice under sea ice.JPG|right|thumb|Anchor ice growing on the sea floor in McMurdo Sound, Antarctica]]
'''Anchor ice''' is defined by the [[World Meteorological Organization]] as "submerged ice attached or anchored to the bottom, irrespective of the nature of its formation".<ref>WMO: ''WMO Sea-ice Nomenclature''. Secretariat of the World Meteorological Organization, 1970.</ref> It may also be called '''bottom-fast ice'''.<ref name=tundra>((Cite web |url=http://www.prairie.illinois.edu/shilts/gallery/shilts-0119.shtml |title="Bottom-fast ice pan domed by spring meltwater influx during breakup" |access-date=2012-11-21 |archive-url=https://web.archive.org/web/20130625073838/http://www.prairie.illinois.edu/shilts/gallery/shilts-0119.shtml |archive-date=2013-06-25 |url-status=dead ))</ref> Anchor ice is most commonly observed in fast-flowing rivers during periods of extreme cold, at the mouths of rivers flowing into very cold seawater, in the shallow [[Subtidal|sub]] or [[intertidal]] during or after storms when the air temperature is below the [[freezing point]] of the water, and the [[subtidal]] in the [[Antarctic]] along [[ice shelf|ice shelves]] or near floating [[glacier tongue]]s, and in shallow lakes.<ref>((cite book|url=http://catalog.hathitrust.org/Record/001273389|title=Ice formation, with special reference to anchor-ice and frazil|author =Barnes, Howard T.|authorlink=Howard Turner Barnes|year=1906|location=NY|publisher=J. Wiley & sons))</ref>

==Types and formation==
===In rivers===
[[Image:GRUNDEIS IMG 6409.jpg|right|thumb|Anchor ice on the ground of river Saale in Jena, Germany]]
Anchor ice will generally form in fast-flowing rivers during periods of extreme cold. Due to the motion of the water, ice cover may not form consistently, and the water will quickly reach its [[freezing point]] due to mixing and contact with the atmosphere. [[Ice platelet]]s generally form very quickly in the water column and on submerged objects once conditions are optimal for anchor ice formation. Anchor ice in rivers tends to be composed of numerous small crystals adhering to each other in small flocculent masses. Anchor ice in rivers can seriously disrupt [[hydro-electric power]] plants by significantly reducing flow or stopping turbines completely.

Another form of anchor ice may be observed at the mouths of [[Arctic]] rivers where fresh water seeps out of the river bed into the ocean up through the sediment. Anchor ice forms if the seawater is below the [[freezing point]] of the river water.

===In lakes===
Shallow [[tundra]] lakes may feature anchor ice with a specific behavior.<ref name=tundra/> Lakes in the southwestern part of [[Nunavut, Canada]] typically freeze down to the bottom when the water level is low. On some cases spring meltwater flows into the lake under the ice cover, which becomes domed leaving a depressed "racetrack" ring around the shore where meltwater accumulates as well. The ice cover remains bottom-fast until the buoyancy force exceeds the freezing bond. At the latter moment the ice cover abruptly breaks off the bottom to form a flat sheet.<ref name=tundra/> In other cases  the anchor ice becomes completely submerged into the meltwater and holes may be melted throughout the ice sheet. When the sheet finally lifts off the bottom, the meltwater accumulated at the surface is jetted through these holes with enough force to create small craters in the lake bottom where it is soft (sand or silt). This downward jet phenomenon was previously described for deltas into the [[Beaufort Sea]], where they were caused by periodic tidal buoyancy of holed ice.<ref name=tundra2>((Cite web |url=http://www.prairie.illinois.edu/shilts/gallery/shilts-0047.shtml |title="Holes in submerged, bottom-fast ice of a small tundra lake during breakup " |access-date=2012-11-21 |archive-url=https://web.archive.org/web/20130625074331/http://www.prairie.illinois.edu/shilts/gallery/shilts-0047.shtml |archive-date=2013-06-25 |url-status=dead ))</ref>

===Formed during storms===
Anchor ice may be formed in the shallow [[intertidal]] or [[subtidal]] during storms in cold weather, when the uppermost layers of the water column are churned up by strong winds or waves. This type of anchor ice can be found primarily in the [[Arctic]], where submerged ice may be observed to completely cover the substrate to depths of up to 2m, with some anchor ice cover observed at more than 4.5m depth.

===In the Antarctic===
((unreferenced section|date = March 2018))
[[Antarctic]] anchor ice is perhaps one of the most interesting phenomena of ice formation in the marine environment. The general mechanism of its formation is commonly assumed as the following:
*Antarctic surface waters are forced to flow below a large, thick mass of floating ice ([[ice shelf]] or [[glacier tongue]]) due to tidal motions or normal ocean circulation.
*The surface water melts the underside of the mass of ice, causing a slight freshening of the water that brings the temperature of the water into [[Thermodynamic equilibrium|equilibrium]] with the in-situ [[freezing point]] at depth.
*The water, at its [[freezing point]] at depth (slightly lower than the freezing point at the surface due to the pressure effect on the freezing point), is [[advection|advected]] from under the floating mass of ice and may rise towards the surface due to a variety of factors.
*As the water rises the in-situ freezing point increases, leaving the water slightly [[supercooling|supercooled]]. [[Supercooling]] is relieved by the formation of microscopic [[ice crystal]]s in the water column.
*Ice crystals may coalesce or adhere to submerged objects, including marine organisms, rocks, man-made structures, or other ice formations such as the [[sea ice]], the [[ice foot]], floating [[glacier]]s or [[iceberg]]s.

Anchor ice crystals in the Antarctic are generally in the form of thin, circular platelets of 2–10&nbsp;cm in diameter. Large masses of irregularly-oriented crystals form anchor ice formations, which may be as large as 4m in diameter when attached to large immovable objects on the sea floor.

Anchor ice that forms on the underside of sea ice is often referred to as [[platelet ice|platelet]] or [[congelation ice]], and can be hard to distinguish from that formed due to the cooling of the sea ice cover by cold atmospheric conditions.

Anchor ice is thought to be relatively common in the Antarctic, due to large [[ice shelf|ice shelves]] that occupy many areas of the continental coast. Studies and observations of anchor ice formation in [[McMurdo Sound]], [[Antarctica]] have shown that the phenomenon regularly causes the formation of ice on the seafloor to depths of approximately 15m, and rarely to depths of approximately 30m.
<div style="text-align: center;">
;Anchor ice in Antarctica
</div>
<gallery mode = packed heights = 200px>
File:Antarctic Ice shelves drive the formation of supercooled seawater.png|Ice shelves can cause the formation of supercooled seawater and anchor ice.
File:Anchor ice platelets on rope.JPG|Anchor ice growing on a rope.
</gallery>

==Biological effects==
[[Image:Scientist and sponges.jpg|thumb|A scientist investigating a sponge on the surface of the Western McMurdo Shelf, McMurdo Sound, Antarctica]]
Especially in the [[Antarctic]], anchor ice has been implicated in drastic zonation of the [[subtidal]] [[fauna]]. Many animals are directly affected by the growth of anchor ice, and certain [[sea sponge|sponge]]s have been shown to readily grow anchor ice and to be damaged by it. Anchor ice may also grow on animate or inanimate objects and lift them from the sea floor. In the [[Antarctic]] this will most likely result in the death of an organism, since during much of the year the ocean is covered by annual sea ice, and the organism is likely to become incorporated into this.

Many organisms have actually been found on the surface of [[ice shelf|ice shelves]] in certain places in the [[Antarctic]], likely due to the anchor icing phenomenon:<ref>((cite journal|author =Dayton, Paul K. |author2=Gordon A. Robilliard |author3=Arthur L. Devries |name-list-style=amp |title=Anchor ice formation in McMurdo Sound, Antarctica, and its biological effects|journal=Science|volume=163|issue=3864|year=1969|pages=273–274|doi=10.1126/science.163.3864.273 |pmid=17790253))</ref>

*Organism accumulates anchor ice as it is bathed in supercooled water.
*Organism becomes positively buoyant due to the accumulation of ice, and is lifted from the sea floor.
*Organism floats to the underside of the [[ice shelf]] or [[sea ice]] cover where it freezes in.
*[[Ablation]] of the surface of the ice cover and additional growth of ice on the underside will result in the organism being transported "through" the ice and "deposited" on the surface, largely intact.

==See also==
*[[Fast ice]]

==References==
((reflist))

==External links==
* [https://web.archive.org/web/20060720212450/http://faculty.gg.uwyo.edu/kempema/vidtour.html Anchor ice formation and rafting in rivers: A video tour]
* ((cite EB1911|wstitle=Ground-ice|volume=12|pages=624–625))

((Ice |expanded))

[[Category:Bodies of ice]]

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